The present invention relates to discharge lamp units, and more particularly to a discharge lamp unit in which a discharge lamp having discharge electrodes confronted with each other in a closed glass envelope is supported by a pair of lead supports protruding from a lamp holder.
A conventional discharge lamp unit of this type is constructed as shown in FIG. 1. A pair of lead supports 3 and 4 serving as conductors are embedded, in an insulating base 2, which is a lamp holder and is made of synthetic resin. The lead supports 3 and 4 support a discharge lamp 5, which is constructed as follows:
A pair of electrodes 6 are held confronted with each other in a closed glass tube 5a. The electrodes 6 are electrically connected to two lead wires 7 which are connected to respective ones of metal supports 8a and 8b. The metal supports 8a and 8b are mounted on the lead supports 3 and 4, respectively. The lead supports 3 and 4 are welded to terminals 9a and 9b, respectively, which are embedded in the insulating base 2 in such a manner that they extend outward from the side of the base 2 opposite to the side where the discharge lamp is mounted.
In assembling the discharge lamp unit, the lead supports 3 and 4 are welded to the terminals 9a and 9b, respectively, in advance, and then the assembly of the lead supports and the insulating base is then molded around the terminals. Thereafter, the metal supports 8a and 8b are welded to the lead supports 3 and 4, respectively, and then the discharge lamp 5 is secured to the metal supports 8a and 8b.
The above-described discharge lamp unit is disadvantageous in the following points: If the spot-welding margins of the terminals 9a and 9b and the lead supports 3 and 4 are not large enough, then the former may not positively welded to the latter. Moreover, if the distance A between the surface of the base 2 of resin and the end faces of the terminals 9a and 9b is not long enough, the end portion of the terminals may be exposed, appearing at the surface of the base 2. In such a case, an electrical discharge may occur between the lead support 3 and the exposed end portion of the terminals 9b. In order to eliminate this difficulty, it is necessary that the wall thickness d of the resin base 2 be sufficiently large. However, since the base 2 is formed by molding synthetic resin, sometimes it includes blowholes 2a, as shown in FIG. 2. The probability that blowholes 2a are formed in the base 2 increases with the wall thickness of the base 2. If blowhole cracks are present in the terminal region of the base 2, then current can flow between the terminals through the blowholes, as indicated by arrows in FIG. 9. That is, the dielectric strength of the base is lowered, and accordingly electrical discharge occurs between the terminals 9a and 9b. As is apparent from the above description, the conventional discharge lamp unit suffers from a difficulty that, in the case where it is intended to increase the dielectric strength by increasing the wall thickness of the base, sometimes blowholes are formed in the base, thus lowering the dielectric strength.